The present invention relates to the recharging of a battery, in particular that of a radiotelephony handset. In order to limit the weight of the handset, the use of lead batteries is avoided and preference is given for example to batteries employing Cadmium-Nickel or NiMH (Nickel Metal Hydride) technology, although they are more delicate. In order to avoid damaging them, they are recharged using a limited current, from a charger comprising a current generator.
A current generator is, by nature, designed to deliver the requisite current, regardless of the voltage of the battery. Hence, a momentarily very low battery voltage cannot cause an excessive inrush of current. However, the voltage of the battery, which rises owing to the recharging current, cannot oppose this current when the battery is recharged. The energy of the recharging current, which is normally converted into chemical energy for recharging the battery, can no longer perform this conversion when all the electrolyte has been converted and the battery is equivalent, in respect of the recharging current, to a heating resistor. The corresponding heating might therefore destroy it if it is not possible to detect the end of chemical recharging state, so as to stop the recharging current.
To detect the end of recharging, it is known practice to monitor the voltage of the battery, which increases during recharging but decreases slightly and temporarily in the transient phase during which, at the end of recharging, the battery begins to become equivalent to a heating resistor.
However, in an environment of use in an apparatus, such as the aforementioned handset, the circuits supplied by the battery are sometimes placed temporarily on standby and reactivated, so that the variations in discharge current thereby induced cause positive or negative jumps in battery voltage. Recharging might therefore be interrupted too soon or, conversely, it might never be interrupted if such a voltage jump masks the temporary fall in voltage, indicative of the end of recharging.
The applicant has contemplated monitoring the absolute level of the temperature of the battery so as to detect the changeover to the xe2x80x9cheating resistorxe2x80x9d mode. However, the discharge current, or else the environmental conditions of the terminal, such as exposure to the sun""s rays, may also cause heating of the battery, which might wrongly be regarded as an end of recharging.
EP-A 0225 789 teaches the detecting of a maximum battery temperature so as to stop the recharging.
WO 96 19860 A teaches the measuring of the battery heating, which results from a specified current, so as to stop the recharging when the variation in heating exceeds a threshold.
The present invention aims to reliably control the recharging of a battery.
Accordingly, the invention relates firstly to a process for controlling the recharging of a battery from a current generator, in which process:
a charging current is applied to the battery is varied cyclically from an initial value,
the temperature of the battery is monitored so as to search for and measure therein a corresponding variation of the temperature, and
the value of the current is reduced if the temperature variation exceeds a target threshold, which process is characterized in that
the value of the charging current is varied cyclically from an initial value and in that it is the corresponding temperature variation which is compared with the threshold is reduced if the temperature variation exceeds a target threshold.
Thus, the variation in current is equivalent to a succession of pulses which is manifested thermally at the battery level by a succession of thermal cycles in phase with these pulses. It is this sensitivity of conversion, or slope, between the variation in current and the variation in temperature which is thus determined so as, if need be, to bring the current to a value such that it does not, by itself, heat the battery by a value exceeding the target threshold value. The value of the ambient temperature does not intervene as such, since it may be constant, and since it is sufficient to deduct it from the temperature measurements so as to deduce therefrom the thermal variation, or since it may drift, but it is equivalent to a variable changing at a different frequency from that of the pulses, hence with no phase relationship with the pulses, this making it possible, eventually, to accurately determine their influence. It is appreciated that the process applies to any battery, be it isolated or incorporated into any type of apparatus.
Preferably, the value of the temperature variation is slaved to the threshold value by adjusting the current in accordance with the deviation between the said temperature values.
It is thus possible to recharge at the maximum current compatible with the target value.
Advantageously, the value of the temperature variation is determined by comparison between a heating slope and a cooling slope.
It is thus possible to circumvent the drifting of the ambient without having to monitor the latter since its overall effect is then zero.
The invention also relates to a device for controlling recharging of a battery configured so as to be supplied by a current charger, comprising
means for providing a target threshold value of temperature variation of the battery, characterized in that it includes
means of variation for varying the recharging current,
sequencer means for cyclically controlling the means of variation, and
means for measuring the temperature of the battery,
means of calculation, linked at input to the threshold-providing means and to the temperature-measuring means so as to determine therefrom a temperature variation value and to control means of adjusting the current in accordance with the deviation between the temperature variation value and the threshold value.